Fix StaticType.flatten() on AnyOfType containing AnyType

[alancai98]

Description

Fixes some previous buggy behavior when flattening an AnyOfType containing AnyType:

AnyOfType(setOf(StaticType.INT, StaticType.ANY))           // union(int, any)
AnyOfType(setOf(StaticType.INT, StaticType.ANY)).flatten() // union(int, any)

With this change, the above will now be:

AnyOfType(setOf(StaticType.INT, StaticType.ANY))           // union(int, any)
AnyOfType(setOf(StaticType.INT, StaticType.ANY)).flatten() // any

Other Information

• Updated Unreleased Section in CHANGELOG: [YES]

• Any backward-incompatible changes? [NO]

• Any new external dependencies? [NO]

• Do your changes comply with the Contributing Guidelines
  and Code Style Guidelines? [YES]

License Information

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

Conformance comparison report

	Base (327ea86)	a66fd7e	+/-
% Passing	92.54%	92.54%	0.00%
✅ Passing	5384	5384	0
❌ Failing	434	434	0
🔶 Ignored	0	0	0
Total Tests	5818	5818	0

Number passing in both: 5384

Number failing in both: 434

Number passing in Base (327ea86) but now fail: 0

Number failing in Base (327ea86) but now pass: 0